Metal sheets such as tin-free steel (TFS) sheets and aluminum sheets that are used as metal can materials for food can containers are conventionally painted for the purpose of improving corrosion resistance, durability, weather resistance, etc. However, the technique used for painting has problems such as its complicated baking process, a long treatment time, and discharge of a large amount of solvents. To solve these problems, resin-coated metal sheets prepared by laminating thermoplastic resin films onto heated metal sheets have been developed as substitutes for the painted metal sheets and are currently used widely for industrial purposes, mainly for beverage can materials.
A problem with the use of a resin-coated metal sheet for a food can container is that, since the contents of the container strongly stick to the inner surface of the container when the contents are taken out of the container, the contents are not easily taken out. This problem is closely related to consumer purchasing intention and is a very important issue in terms of ensuring the consumer purchasing intention. Nevertheless, there are very few studies on improvement in the ease of releasing contents from the conventional resin-coated metal sheets (their content releasability).
Accordingly, the present inventors have conducted extensive studies to secure content releasability and found that, when the contents include a large amount of fatty ingredients (e.g., a mixture of meat, eggs, and carbohydrates), good content releasability can be secured by adding a specific wax to a polyester resin such that the wax is present on the surface of the resin. Thus, the inventors have filed Patent Literature 1.
The inventors have further conducted studies and found that, when the interfacial free energy between water and the surface of a resin layer that is to be disposed on the inner side of a container is 30 mN/m or more, excellent content releasability can be obtained even for highly sticking contents (such as luncheon meat) with a high protein content. The inventors have also found that the above surface state can be obtained by adding a fatty acid amide and filed Patent Literatures 2 and 3. The effects of the addition of the fatty acid amide is obtained when the amount added is 0.5 mass % or more. Specifically, it is known that, to obtain the effects sufficiently, the fatty acid amide must be added such that its content is 5.0 mass % or more in at least the uppermost layer of the resin layer that is in contact with the contents.
Desirably, food can containers have inner surfaces with content releasability and outer surfaces that are not adversely affected when the containers are subjected to high-temperature sterilization treatment such as retort treatment. When a resin coating (laminated coating) is applied to the outer surface of a can by thermal-fusion bonding of an ordinary polyester resin film, a problem arises in that the polyester resin film is discolored when subjected to high-temperature sterilization treatment such as retort treatment. This is a phenomenon in which fine air bubbles are formed in the resin layer and scatter light to thereby cause a cloudy appearance. In Patent Literature 4 by the present inventors, design is ensured even after retort sterilization treatment by forming a resin layer on the side which, after the formation of a container, becomes the outer side of the container using a polyester composition prepared by mixing a polyester including ethylene terephthalate as a main repeating unit and a polyester including butylene terephthalate as a main repeating unit at a specific ratio.